How Do Ships Work?

The power of the water

The sink of the oil tank Prestige caused one of the worst enviromental disaster on the shores of Spain. The ships broke in two parts

For millennia, people had been making ships of wood. Than, around 1790, the first iron ship was made. People thought iron would sink, as it is denser than the wood. But the floating capacity depends on the ratio between weight and volume. No matter the weight of a ship, it will float if its volume is large enough. A solid block of iron does not float, but the same block can be transformed into a floating ship of the same weight.

When an object is put onto the water, it displaces a certain volume of water. That water is maintained on place by an ascendant force which will act on the object placed on the water and that's why it will decrease the apparent weight of the object with the weight of the displaced water volume.

Thus, a sunk object will experience an ascendant force that grows gradually, matching the weight of the displaced water. In the case of an iron bar, the ascendant force will be too weak to sustain it, and the bar will sink. On the contrary, with the same iron mass, but with a voluminous shape, enough water will be displaced making the object float at the level that displaces its own water weight.

If we load the boat with a small cargo, this will push the boat downward till the body of the boat displaces a sufficient water surplus to ensure the ascendant force necessary for sustaining the load. The boat will float at a level where the weight of the displaced water matches the total weight of the floating body. If the cargo is too heavy, the boat does not reach a point to displace enough water for sustaining and it will sink.

On an agitated sea, the ships experience huge solicitations. That's because the ascendant force on each part of the ship depends on the height of the waves in that point. Where the back of the wave is found, the effective displacement is the largest, and the ascendant force the highest. The lowest ascendant force acts in the concavity of the waves. On agitated sea, the ascendant force varies along the ship body and changes permanently; ships must be designed to stand such forces.

The worst situation is when the ship advances on waves whose distance between the peaks equals the ship's length. If at each end of the ship a wave is found, and the middle overlaps a concavity, the ascendant forces at the ends will be much larger than in the middle, and the middle of the ship tends to go down. If the middle of the ship is over a wave, and the ends over concavities, the ends tend to go down. In extreme conditions, this can break the ship in halves.

Most ships are too small to be affected by these problems. But long ships are exposed, while large cargoboats, like tank ships and raw material transporting ships, are wide enough, so that their length and width have reasonable proportions.

Ships are also exposed to the inward water pressure. This pressure increases with the depth and has a strong effect upon ships with deep draught (the distance between the surface of the water and the bottom of the ship's body). The body of such ships must be very strong to stand water pressure. Other forces can cause the vibration of the ship's body, due to the balance and repeated blows of the waves.

While the ship goes up and down on an agitated sea, the prow (fore part) experiences great variations of water pressure. When descending, the body is pushed inwards. When the prow ascends, the pressure ceases. The resulting movement causes vibration, due to the balance. It installs in the whole body of the ship, but it is stronger in the prow. That's why the ship's body is strengthened at the prow.

On agitated sea, the prow also receives repeated wave blows. The prow raises and collapses on the water repeatedly. Ships exposed to repeated blows of the waves are usually strengthened on about one third of the length from the prow backward.

When a ship receives waves from one side, the depth of the water on both sides changes continuously. The different forces produced along the ship's body tend to deform it. To counteract this effect, called push, the inner structure of the ship is hardened in the points where the traverses along the ship's body join the sides.

The relative stability of the ship is given by the relative position of three imaginary points: weight center, floatage center and metacenter. The ship's weight would push downward from the point called weight center. Floatage would be a force acting upward from a point called floatage center. When the ship stays right, the floatage center would be just under the weight center on a median line.

No matter if winds or waves tilt the ship, the weight center remains unchanged. But the floatage center moves on the median line to the lower part of the ship. The weight force directed downwards and the floatage force acting upwards bring back the ship on the right position. If the ship inclines transversally, the vertical line acting on the floatage meets the median line in a point called metacenter. As much as this is over the weight center, the ship is stable and returns on its right position after the force that caused the tilt is gone. If the two points coincide, the floatage and weight will counteract one another. There will be no force to redress the ship, which will remain tilted.

If the metacenter is found under the weight center, the ship will be unstable. Once starting to tilt, the action of the weight and floatage will further till the ship, overturning it. Unusual conditions, like overload, can produce instability. Most ships wait the passing of the storms or change the route. Lifeboats are projected to redress by themselves when capsized.

Roll is a ship's action of rocking from side to side, caused by wind or waves. Besides inducing tensions in the structure, the roll is unpleasant for persons on the board. The roll is more evident in very stable ships, designed to resist to overturn, as powerful forces act to correct any tilt. To reduce the roll, appliances called stabilizers are mounted.

Pitch is an oscillatory movement of the ship in longitudinal plan, effectuated along its transversal ax. Each ship has a period of natural pitch, depending on its size, shape and weight. The overall pitch is the result of natural pitch, wave length, and the ship's speed.

If a ship advances with a given speed on waves of a certain length, it will meet waves on intervals similar to its pitch period. Like the cradle of a baby is slightly pushed at equal intervals, the same way the ship will oscillate violently forward and backward. This effect is called resonance, and the slightest impulses can cause a dramatic effect if there is a certain interval between successive impulses. To minimize the pitch effect, ship builders make tests with waves on models of scale-made ships. If necessary, the builder will modify the ship's structure to be sure that the pitch is maintained at a tolerable level.

To advance on the water, a ship must confront resistance forces due to friction, waves, shape, eddies and accessories (like rudder or propeller). Resistance due to friction is caused by the rubbing effect between the ship's body and water. The waves' resistance is caused by the fact the ship generates waves while advancing. Resistance due to shape is a backward oriented force, caused by a zone of low pressure created in the water in the back of the ship during its advance. Resistance due to eddies is caused by the ship's body generating circular currents in the water.

The overall resistance of the ship is estimated by its builders on a scale-made model. Still, the engine's power must overcome the calculated value, as no propulsion system is 100 % efficient. There are losses of engine power and propeller power. Ringed propellers decrease the fuel consume, being more efficient. Bumped prows can reduce the water flow and resistance (and the consumed fuel). Small ships and boats are acted by Diesel engines, but most large ships are acted by vapor turbines.